It is known to produce acrylic esters by carrying out a transesterification reaction between a light alkyl acrylate (known as light acrylate) and a heavy alcohol.
This reaction is an equilibrated catalyzed reaction with generation of light alcohol, according to the formula (I):CH2═CH—COOR1+R2—OHCH2═CH—COOR2+R1—OH  (I)
It is necessary to remove the light alcohol produced during the reaction in order to shift the equilibrium in the direction of the production of the acrylic ester.
This reaction is generally accompanied by side reactions which produce impurities which it is necessary to remove for the purpose of obtaining the acrylic ester with a high purity which meets the technical requirements related to its final use as monomer for manufacturing polymers which can be used in numerous fields of application.
Furthermore, for obvious economical reasons, the products of economic value present in the crude reaction mixture, in particular the unreacted reactants and the catalyst, are as far as possible recycled in the process.
For these purposes, a separation/purification process comprising a combination of distillations, extractions and/or separations by settling is generally carried out, which process is simultaneously relatively complex to carry out, in particular as a result of the presence of azeotropic mixtures, and expensive energetically.
The applicant company has been concerned more particularly with the synthesis of 2-propylheptyl acrylate (alkyl acrylate with a branched C10 chain) from a light acrylate and 2-propylheptanol (known as 2-PH), it being possible for this monomer to exhibit advantageous properties in the field of coating materials, paints, inks and adhesives.
According to the document JP05-070403, 2-propylheptyl acrylate can be obtained by direct esterification of acrylic acid or by transesterification of a light acrylate, such as methyl acrylate, with 2-propylheptanol in the presence of p-toluenesulfonic acid as catalyst and of a polymerization inhibitor, according to methods well known to a person skilled in the art. The methanol formed is continuously removed and the reaction is followed by a distillation under reduced pressure by way of purification of the reaction product obtained. The document JP05-070403 does not clearly mention the stages of purification to be carried out or the level of purity to be expected for the monomer for the purpose of the use thereof.
Various transesterification processes for producing C2-C12 alkyl acrylates have already been described in the prior art.
Mention may be made, for example, of the document U.S. Pat. No. 7,268,251, in which the reaction effluent stream from the transesterification is treated in the following way:                either most of the acrylic ester desired is first of all separated and is subsequently isolated from the catalyst used by distillation (separation of catalyst),        or the catalyst used is first of all isolated by distillation (separation of catalyst) and subsequently most of the acrylic ester is separated,        and subsequently the compounds having a lower boiling point than that of the desired acrylic ester are separated by distillation from the mixture obtained (separation of the low-boiling-point substances) and subsequently the acrylic ester is distilled (distillation in the pure state).        
This process requires the use of at least four distillation or rectification columns, including an evaporator in order to separate the catalyst, generally a titanium alkoxide.
Even if the process described in the document U.S. Pat. No. 7,268,251 relates to the manufacture of alkyl acrylates by transesterification starting from an alkyl acrylate and from an alcohol exhibiting a chain length greater by at least one carbon with respect to the alkyl chain of the starting acrylate, this process is illustrated only with the manufacture of dimethylaminoethyl acrylate from dimethylaminoethanol and from methyl acrylate or ethyl acrylate in a cascade of two reactors.
It turns out that the process described in the document U.S. Pat. No. 7,268,251 is complicated to carry out on an industrial scale, as a result of the optimization of the operating conditions of the sequence of the four distillation/rectification components, in order to obtain a product of high purity and a satisfactory productivity.
The document U.S. Pat. No. 6,977,310 describes a process for the continuous manufacture of (meth)acrylic acid alkyl esters from methyl (meth)acrylate and a C2-C12 alcohol in the presence of a tetraalkyl titanate as transesterification catalyst. This process consists in subjecting the reaction mixture to a distillation under reduced pressure which separates the readily volatile compounds (unreacted reactants); then, the resulting fraction exiting at the column bottom, comprising the ester produced, the catalyst, the polymerization inhibitors and the high-boiling-point byproducts, is sent to a vacuum distillation stage. This vacuum distillation stage comprises in particular a film evaporator, combined with a distillation column, for complete removal of the high-boiling-point products in the ester produced. The ester produced is recovered at the top with a high purity.
According to this process, illustrated solely with the manufacture of butyl methacrylate or isobutyl methacrylate, the desired methacrylate occurs in the bottom stream from the first column for distillation under reduced pressure, before being separated from the catalyst and then purified.
The processes of the prior art using alkyl titanate, such as tetraethyl titanate, tetrabutyl titanate or tetra(2-ethylhexyl) titanate as transesterification catalyst, are not directly applicable to the manufacture of long-chain alkyl acrylate, for example 2-propylheptyl acrylate, by transesterification reaction of a light acrylate with 2-propylheptanol. This is because the transesterification of titanates, either with the light alcohol released during the reaction (methanol or ethanol) or with the starting 2-propylheptanol, brings about the appearance of impurities, such as butyl acrylate or 2-ethylhexyl acrylate, in the reaction mixture or in the light ester/light alcohol azeotropic mixture and complicates the purification of the 2-propylheptyl acrylate.
The applicant company has attempted to solve the various problems of the abovementioned processes, in particular those related to the use of 2-propylheptanol in the transesterification reaction catalyzed by an alkyl titanate, and has discovered a simplified manufacturing process for producing 2-propylheptyl acrylate of very high purity with a high yield, while including the recycling of the products of economic value, such as the unreacted reactants and the catalyst, thus exhibiting a productivity compatible with manufacture on an industrial scale.
The solution provided consists in using ethyl titanate in solution in 2-propylheptanol or 2-propylheptyl titanate as transesterification catalyst and in employing a purification line comprising only a single distillation column and a film evaporator.
Furthermore, the inventors have found that tin derivatives, in particular dibutyltin oxide, as transesterification catalysts also make it possible to overcome the disadvantages of the abovementioned processes and can be used to produce 2-propylheptyl acrylate in a transesterification process comprising a simplified purification line with just one distillation column and a film evaporator.